Overload protection unit for readjusting switching elements

ABSTRACT

Method for regulating a plurality of output voltages of a converter, of which a first output voltage controls the converter and further output voltages are kept constant by readjusting elements, it being the case that when a prescribed voltage difference across the readjusting element is exceeded, the converter is controlled by a control loop, proceeding from the respective readjusting element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an electrical power supplyhaving a plurality of outputs and more specifically to an overloadprotection circuit.

2. Description of the Related Art

Switched-mode power supplies having a plurality of output voltages areoften implemented by means of transformers which contain a plurality ofoutput windings. Since in the case of this principle only one outputvoltage can be regulated directly via the converter, the remainingoutputs have to be regulated indirectly. For this purpose, linearreadjusters are used which bring the unregulated voltage UA_(X) of anoutput winding to the desired value UA_(X) . This principle isrepresented in FIG. 1.

In general, the readjusting circuit contains a current limiter which, inthe event of a fault, such as in the event of a short circuit, protectsthe connected load and the readjuster itself. In most applications, avirtually vertical current limiting characteristic curve is desired. Aprofile of such a current limiting characteristic curve is representedin FIG. 1a. The associated power loss P_(v), which is converted at thelinear regulator as a function of the load current, is represented in asecond characteristic curve I_(load) /P_(V) in FIG. 1b. For the purposeof simplification, for this representation the unregulated outputvoltage UA_(X) is assumed as a constant U₀.

As can be seen from the course of the characteristic curve, the powerloss rises sharply in current-limiting operation. A series transistor,which is integrated in the linear regulator, must therefore be protectedagainst overheating, that is to say thermal destruction.

In order to avoid the overheating of the series transistor, anover-dimensioned heat sink has previously been used or a switching-offof the circuit arrangement in the case of overload has been obtained.

If the maximum power loss occurring P_(max) =U₀ ×I_(KS) is sufficientlysmall, this can easily be dissipated by means of a somewhatover-dimensioned heat sink. The series transistor, which converts thepower loss into heat, can thus be protected effectively.

For the case in which P_(max) is relatively large, the simple solutionusing the over-dimensioned heat sink is no longer tolerable, since thisapproach would lead to a large-volume and expensive heat sink.

Using a temperature sensor (for example an NTC or bimetallic switch),the temperature of the transistor can be monitored. If the temperatureexceeds a limiting value, the readjusting circuit is switched off untilit has cooled down once more to a sufficient extent. This procedure isrepeated until the cause of the overheating, which can be a shortcircuit in the load circuit, for example, has been eliminated.

One significant disadvantage of this protection circuit is that a sensorhas to be thermally coupled to the transistor which is to be protectedin the readjusting circuit, that is to say the two components must bemounted close alongside each other on a heat sink. Since the mountingcannot be carried out in an automated fashion, this solution isrelatively expensive and, in addition, requires space on the heat sink.

A method which is often used to protect the transistor arranged in thereadjusting circuit from overload is to make the desired value of thecurrent regulation dependent on the output voltage. The advantage ofthis principle is that the short-circuit current is considerably smallerthan the current at which the limiting sets in. Since at a relativelysmall output voltage, that is to say a relatively large voltage acrossthe transistor, the current is reduced, the power loss is considerablysmaller than in the case of irreversible current limiting. However, thismethod is associated with the disadvantage that it cannot be employed inan unrestricted manner.

In the case of loads which need a constant current, such as for exampleremotely-fed telephones, it may be that the output voltage cannot "runup". This effect occurs when at the beginning of running up (UA_(X) =0),the connected loads require a higher load current than the short-circuitcurrent. Since this does not permit a reversible characteristic curve,the loads attempt to establish the desired current by lowering theirload impedance. The result is that the loads pass into the short-circuitcurrent range, and the readjuster never leaves the short-circuit point.

SUMMARY OF THE INVENTION

The present invention is based on the object of providing a simpleprotection circuit whilst avoiding the disadvantages listed above.

The achievement of this and other objects and advantages is apparentfrom a method of regulating a plurality of output voltages of aconverter, of which a first output voltage controls the converter via afirst control signal and at least one further output voltage is keptconstant by a readjusting element, wherein, if a prescribed voltageacross the readjusting element is exceeded, a second control signal isgenerated and passed on to a decoupling arrangement via a regulatingelement, in addition, the control signal formed from the first outputvoltage is present across the decoupling arrangement and a controlsignal is passed on to the converter via the decoupling arrangement.

In addition to the advantage that the circuit arrangement manageswithout an over-dimensioning of a heat sink, the invention isdistinguished by a further advantage that the expenditure in terms ofcircuitry for the protection circuit is low.

BRIEF DESCRIPTION OF THE DRAWINGS

Further special features of the invention become evident from thefollowing more detailed explanations relating to the drawings, in which:

FIG. 1 is a block circuit diagram which shows a known circuitarrangement of a converter having a plurality of output voltages,

FIG. 1a is a graph which shows a current/voltage characteristic curve,

FIG. 1b is a graph which shows a current/power characteristic curve,

FIG. 2 is a basic block diagram of an overload protection circuitaccording to the invention,

FIG. 3 is a circuit diagram which shows a circuit configuration of anoverload protection circuit, and

FIG. 4 is a graph which shows a characteristic curve of a regulatedoutput voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.

In the case of the circuit principle according to the invention, use ismade of the fact that the input voltage UA_(X) for the readjuster LR isnot fixed but can be influenced. With the aid of a third control loop(see FIG. 2), which intervenes in the regulation of the directlyregulated output voltage, the voltage across the series transistor of areadjuster LR, in particular of a linear readjuster, can thus belimited, as required, to a prescribed value U_(DIF), MAX. This principleis represented in FIG. 2.

The additional control loop can take over the monitoring of theconverter U via a control signal S₂. In normal operation, U_(DIF) issmaller than U_(DIF), MAX. An integrated positive control deviation Dleads to the control signal S₂ finding itself at the upper stop U_(max)of the control voltage and hence cannot intervene in the regulation ofthe directly regulated loop. In this case, the converter U is controlledby the control signal S₁ derived directly from a first output voltageUA₁.

However, if the voltage across the linear regulator LR exceeds U_(DIF),MAX, the negative control deviation D leads to the output signal of anintegrator I becoming smaller. If the voltage value of the controlsignal S₂ falls below the voltage value of the control signal S₁, theU_(DIF) regulator takes over the monitoring of the converter U (S=S₂).From this time forward, the voltage across the readjusting transistor ofthe linear regulator LR also begins to sink to the maximum valueU_(DIF), MAX.

Since the transistor in the readjusting element LR_(n) does not sufferany damage in the case of increased losses for a short time, with theaid of U_(threshold) (U_(s)) and the integration time constants IT_(I)of the integrator I, it is possible to set a power E which may beadditionally converted for a short time in the transistor of thereadjusting element LR_(n). Depending on the setting of a working pointof the directly regulated loop (voltage value of S₁), the power E to beused lies between the limits E_(MIN) and E_(MAX). These are defined asfollows:

    E.sub.MIN =T.sub.I *U.sub.threshold *I.sub.current limiting

    E.sub.MAX =T.sub.I *(U.sub.threshold +U.sub.Max -U.sub.Min)*I.sub.current limiting

Allowing increased losses for a short time across the transistor of thereadjusting circuit of LR_(n) is associated on the one hand with theadvantage that the control loop does not respond in the case of thesmallest disturbances and, on the other hand, it enables unimpeded"running up" of the directly regulated loop.

The realization of the overload protection in terms of circuitry isrepresented in FIG. 3. The resistors R1 to R5 coupled to the operationalamplifier OPV generate the formation of a difference and a comparisonwith a desired value V_(REF). The integration time constant IT₁ of theintegrator I may be set by means of the resistors R1, R3 and, R6 and thecapacitor C. The threshold voltage U_(threshold) is realized by means oftwo decoupling diodes D₁ and D₂, with the aid of the zener diode Z andthe selection of a level MIN. In the circuit arrangement shown, thecontrol of the converter U is carried out by an optocoupler, atransmitting diode FD and a bias resistor R7 being represented for thispurpose. Using the transistor Q, the divider SPE can be detuned and asufficiently small voltage U_(DIF) (<U_(DIF), MAX) can be simulated forthe voltage U_(DIF), MAX regulation. The U_(DIF), MAX regulation canthus be switched inactive. This can be necessary if the voltage U_(DIF)across the readjuster also exceeds the value U_(DIF), MAX in the case ofa low load (small output current).

An experimental check of the overload protection circuit was carriedout, by way of example, on a power supply subassembly which supplies theoutput voltages -54 V and +52 V. The -54 V output is directly regulatedand the +52 V outputs readjusted. The result is summarized in thediagram in FIG. 4.

Represented on the vertical axis is the input voltage UAX of thereadjusting element LR_(n), the output voltage UA_(X) of the readjustedloop being plotted on the horizontal axis. The distance between themeasured curve and the 1:1 straight line drawn in is, in this case, thevoltage U_(DIF) across the transistor of the readjusting element LR. Themeasured voltage in the marked region, that is to say the voltage acrossthe transistor, never becomes greater than a preset value (in theexperiment about 15 V).

The most significant features of the readjusting circuit can besummarized as follows:

No realization in terms of circuitry of a reversible characteristiccurve of a current limiter is necessary.

Together with a vertical current limiting characteristic curve, thepower is limited to P_(MAX) =U_(DIF),MAX *I_(current) limiting.

The directly regulated loop is "driven down" only to the extentnecessary. The protection for a readjusting circuit LR_(n) only acts ifthe power loss P_(MAX) is exceeded.

Via the threshold voltage U_(s) and the integration constant IT₁, it ispossible to set the power loss E which may be additionally converted fora short time.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

I claim:
 1. A method of regulating a plurality of output voltages of aconverter, comprising the steps of:controlling the converter by a firstoutput voltage via a first control signal; keeping constant at least onefurther output voltage by a readjusting element; generating a secondcontrol signal if a predetermined voltage across the readjusting elementis exceeded; passing the second control signal on to a decouplingarrangement via a regulating element; providing the first control signalacross the decoupling arrangement; and passing one of said first andsecond control signals on to the converter via the decouplingarrangement.
 2. A method according to claim 1, wherein said readjustingelement operates in voltage regulating operation.
 3. A method accordingto claim 2, further comprising the steps of:if the predetermined voltagepotential across the readjusting element is exceeded, the voltagepotential is provided with a negative sign and passed on to a summingcircuit; applying a maximum permissible voltage difference across thereadjusting element to a further input of the summing circuit; andpassing a sum signal on to a circuit input of a regulating element.
 4. Amethod according to claim 3, further comprising the step of:under lowload in the voltage regulating operation of the readjusting element,passing an auxiliary voltage on to the summing circuit, said auxiliaryvoltage being slightly smaller than the maximum permissible voltagedifference across the readjusting element.
 5. A method according toclaim 1, further comprising the step of:passing on only one of the firstand second control signals by the decoupling arrangement to theconverter.
 6. A method according to claim 1, further comprising the stepof:limiting voltage values of the control signals at an input of thedecoupling arrangement are in each case within a predetermined valuerange.
 7. A method according to claim 1, further comprising the stepof:using an integrator as said regulating element.
 8. A method accordingto claim 1, wherein said readjusting element operates in currentregulating operation.
 9. A method as claimed in claim 1, wherein saidfirst control signal is obtained in a first control loop and said secondcontrol signal is obtained in a second control loop.
 10. A method asclaimed in claim 1, further comprising the step of:setting a permittedshort term power which may be converted using an integration timeconstant and a threshold value.
 11. A circuit arrangement for limitingpower loss in a readjusting element, comprising:a first voltage divideris arranged at an input and a second voltage divider arranged at anoutput of the readjusting element; an operational amplifier as anintegrator and connected to taps of the first and second voltagedividers; a resistor connected between a first input of the operationalamplifier and a reference potential; a transistor having a collectorconnected to a second input of the operational amplifier and to the tapof the first voltage divider; and a converter having an input connectedto an output of said operational amplifier for receiving one of firstand second control signals.
 12. A circuit arrangement according to claim8, further comprising:a decoupling arrangement formed from first andsecond diodes, the first and second control signals being present oncathodes of the diodes; and an optocoupler having a photodiode with acathode connected to a line connecting anodes of the diodes.
 13. Acircuit arrangement according to claim 11, further comprising:anoptocoupler between an output of the decoupling unit and an input of theconverter.